The need for monolithic integration of microelectromechanical (MEM) devices with complementary metal-oxide semiconductor (CMOS) devices is extremely important for successful implementation of high frequency active MEM resonators. Monolithic integration of these devices can provide basic RF and mm-wave building blocks with high Q, small footprint, and low power for use in wireless communication, microprocessor clocking, navigation, and sensing applications. However, the majority of MEM resonators require a release step to freely suspend the moving structures. This necessitates complex encapsulation and packaging, causing fabrication to be restricted to MEMS-last or Back-End-of-Line (BEOL) processing of large-scale devices. Additional discussions of these techniques are described in “Technologies for cofabricating MEMS and electronics,” G. K. Fedder, R. T. Howe, T.-J. King Liu, E. P. Quevy, Proceedings of the IEEE, 96 (2), 306-322 (2008), which is incorporated by reference herein for this purpose.
Many modern MEMS resonators are based on released suspended structures, which have free boundaries on all surfaces except where anchors exist for suspending support. A small class of acoustic resonators are partially released, for example, in the case of surface mounted FBAR resonators as discussed in “Thin film resonators and filters” K. M. Lakin, IEEE Ultrasonics Symposium, 895-906 (1999), which is incorporated by reference herein for this purpose, and which will be discussed in greater detail below. In these released resonators, flexural mode, longitudinal mode, shear, or surface waves are commonly excited to form acoustic resonance in the cavity created by free-free acoustic boundary conditions. Among these resonant structures, longitudinal mode resonators have been demonstrated with the highest f-Q products at 5˜6×1013 GHz and have even reached mm-wave frequencies, as described in “Internal Dielectric Transduction of a 4.5 GHz Silicon Bar Resonator,” D. Weinstein, S. A. Bhave, IEDM 2007, 415-418 (2007), which is incorporated by reference herein for this purpose. In high frequency longitudinal resonators, loss mechanisms such as thermoelastic damping and phonon-phonon scattering are much smaller for longitudinal modes. Moreover, the longitudinal mode requires relatively simplified theoretical analysis and simulation, therefore making the design methodology of the resonator easily traceable. Examples of resonators based on longitudinal mode include: an Internal Dielectric Resonator, as described in U.S. Pat. No. 6,841,922, which is incorporated by reference herein for this purpose; and a Resonant Body Transistor, as described in “The resonant body transistor,” Dana Weinstein and Sunil A. Bhave, Nano Letters 10(4) 1234-37 (2010), which is incorporated by reference herein for this purpose.
To achieve a high-yield, low cost, robust fabrication process, and enable monolithic integration of MEMS resonators into CMOS technology, resonators may be fabricated unreleased, i.e., enclosed in one or more solid material layers. Because the boundaries for the resonator are no longer “free”, nor “fixed”, however, the resonant cavity is poorly defined and a great amount of energy is leaked in the form of acoustic waves propagating through the boundaries of the resonator. As a result, building unreleased resonators with performance comparable to released resonators presents a number of challenges, e.g., reducing energy leakage and maintaining a high quality factor (Q).
One of the greatest benefits in building unreleased resonators is the ease of integration with modern CMOS technology, for example, in conjunction with Resonant Body Transistors (RBTs) with internal dielectric drive and Field Effect Transistor (FET) sensing up to 37 GHz, as described in U.S. Patent Application Publication 2011/0024812, Feb. 3, 2011, and “Acoustic resonance in an independent-gate Fin FET,” D. Weinstein, S. A. Bhave, Hilton Head 2010, 459-462 (2010), which are incorporated by reference herein for this purpose. Fabrication of these RBTs is very similar to that of independent-gate FinFETs, which is a candidate for the next generation transistor, and it can enable amplification of the mechanical signal without the introduction of electrical parasitics. Development of unreleased RBTs at the transistor level of the CMOS stack will enable direct integration into front-end-of-line (FEOL) processing, making these devices an attractive choice for on-chip signal generation.